1. Field of the Invention
The present invention relates to a printed circuit board (PCB), and particularly to a PCB with a high density layout of transmission lines.
2. Description of Related Art
Differential pairs are often used in a multi-layer PCB to transmit signals. Referring to FIG. 2, a conventional PCB has edge-coupled differential pairs 17, 18 defined in signal layers 12 and 15 respectively to transmit signals, the differential pair 17 includes two transmission lines 171 and 172, the differential pair 18 includes two transmission lines 181 and 182. In order to achieve high signal quality, the layers 11, 13 above and below the differential pair 17 are set as reference layers, and the layers 14, 16 above and below the differential pair 18 are set as reference layers. In the PCB of FIG. 2, there are four layers 11, 13, 14, and 16 acting as reference layers, only two layers 12 and 15 acting as signal layers.
Referring to FIG. 3, another conventional PCB has edge-coupled differential pairs 27, 28 and 29 defined in signal layers 22, 23, and 25 respectively. A reference layer 21 is disposed above the differential pair 27, a reference layer 24 is disposed below the differential pair 28, and reference layers 24 and 26 are disposed above and below the differential pair 29 respectively. In the PCB of FIG. 3, there are three layers 21, 24, and 26 acting as reference layers, and three layers 22, 23 and 25 acting as signal layers. Because in the conventional PCB, the edge-coupled differential pairs are used to transmit signals, there must be at least one reference layer disposed near the signal layer where the differential pairs are defined to obtain good signal quality, at least one inner layer of the PCB is occupied as the reference layer, therefore the density of the layout of transmission lines is not maximized.
What is needed, therefore, is a PCB with a high density layout of transmission lines.